Base mat assembly and method of constructing the same

ABSTRACT

A precast reinforced concrete base mat element, a base mat assembly and method for constructing a base mat assembly from a plurality of the base mat elements are described. The base mat element comprises a concrete body having first and second conduits extending there through in first and second generally perpendicular directions and a liner plate anchored in the concrete body. Preferably the concrete bodies have interlocking surfaces formed there on. A plurality of the base mat elements may be juxtaposed together with the interlocking surfaces interlocking with one another and with the tensile members extending through the first and second tensile conduits and being post-tensioned and anchored about the base mat elements to clamp the base mat elements together. The liner plates may then be welded together to form a a generally planar membrane. In one embodiment, at least one of the first and second tensile conduits is located above and one below the horizontal center planes of the concrete bodies to limit the base mat elements from displacing vertically relative to one another.

TECHNICAL FIELD

The present invention relates generally to LNG (liquefied natural gas) tanks and methods for constructing LNG tanks, and more particularly, to base mat foundations for containment tanks which are used to support LNG tanks.

BACKGROUND

LNG full containment tanks typically comprise an inner primary container (hereinafter “inner tank”) for storing LNG and an outer container (hereinafter “outer tank”) which is designed to contain leaks from the inner tank. Large storage tanks, such as those used with LNG full containment tanks, are typically sensitive to differential settlement in the foundations supporting the inner tank which stores LNG. Consequently, standard practice is to build a rigid foundation with a flat top surface supporting the bottom of the inner tank. For example, the foundation can be a grade supported reinforced concrete mat, a deep foundation with a mat resting on the ground, or an elevated slab foundation. In cases where very stiff soil or soft rock is present at a construction site, a grade supported mat is the preferred type of foundation. The conventional design for the foundation mats utilizes cast in place reinforced concrete.

Construction using an assembly of precast concrete elements is a known approach when construction time at a site needs to be minimized and when site conditions and locations are problematic for casting in place structures. Assemblies made of precast elements are often used for temporary and, in some cases, for permanent pavement.

However, application of precast elements in LNG tank foundations is not a conventional approach. Such foundations would be subject to high forces at connections which makes design and construction of such connections very difficult. Only small allowances can be made for vertical differential displacements across a base mat when an inner primary container or inner LNG tank is to be supported upon an assembly of precast elements forming the base mat. Grade supported mats provide special support to a large tank bottom so as to minimize differential settlements. If a base mat is to be constructed from precast concrete elements, the forces that will occur due to differential settlement or voids in the underlying soil will tend to separate and displace the base mat elements from one another. It is challenging to provide sufficient vertical stiffness to precast base mat elements forming a base mat while keeping the thickness of the base mats within an economically acceptable range.

SUMMARY

A precast reinforced concrete base mat element, a base mat assembly or foundation and method for constructing a base mat assembly from a plurality of the base mat elements are described. The base mat element comprises a liner plate and concrete body having first and second tensile conduits extending there through. Preferably the tensile conduits extend in first and second generally perpendicular directions. The liner plate is anchored or cast into the concrete body. Ideally the base mat elements have interlocking surfaces formed thereon.

A plurality of the base mat elements may be juxtaposed together. The tensile members extend through the first and second tensile conduits and are anchored and clamped about the base mat elements utilizing anchors with a planar membrane surface being formed by the liner plates. The liner plates may be welded together to form a membrane which is generally flat and which utilizes the liner plates as a vapor barrier. Ideally, if interlocking surfaces are provided on the base mat elements, the interlocking surfaces can interlock with one another to limit displacement of the base mat elements relative to one another. Preferably, the base mat elements interlock in both horizontal and vertical directions. In one embodiment, the first and second tensile conduits are located above and below the horizontal center plane of the concrete body so that when tensile members are passed through and post-tensioned and anchored about the base mat elements using anchors, the clamped base mat elements resist relative displacement between one another due to the clamping force provided by the post-tensioned tensile members and the additional stiffness provided by the membrane.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features and advantages of the present invention will become better understood with regard to the following description, appended claims and accompanying drawings where:

FIG. 1A is a perspective view of an inner tank being constructed within an outer containment tank as the roof of the outer tank is being constructed;

FIG. 1B is a perspective view of a completed outer tank, in partial cutaway, showing a base mat assembly, a ring beam, a shell or tank wall and a roof mounted atop the shell as well as the inner tank;

FIG. 2 is a plan view of the base mat assembly or foundation and a ring beam or ring beam foundation which are to be joined by a cast in place concrete connection (not shown);

FIG. 3A is a plan view of a pair of base mat elements juxtaposed and interconnecting with one another;

FIG. 3B is a perspective view of a liner plate and attached J-hooks which are to be cast within a concrete body when making a reinforced concrete base mat element;

FIG. 3C is a perspective view of a base mat element, including concrete body, showing a pair of tensile members passing through tensile conduits disposed within the concrete body;

FIG. 3D shows a plurality of base mat elements juxtaposed in lateral and longitudinal directions with tensile members and cooperating anchors clamping the base mat elements together in the lateral and longitudinal directions;

FIG. 4 shows an exemplary pile with L-shaped brackets mounted thereon which can be used to support base mat elements or ring beam elements at predetermined heights so that the base mat elements or the ring beam elements can be readily welded together;

FIG. 5 shows a matrix of piles which can be used to support base mat elements which can then be welded together to form the base mat assembly;

FIG. 6A is a plan view of a pair of cooperating juxtaposed ring beam elements;

FIG. 6B is a perspective view of a liner plate welded to a thicker base plate and attached J-hooks welded beneath the plates which are used in casting a reinforced concrete ring beam element;

FIG. 6C is a perspective view of a reinforced concrete ring beam element including the liner plate and base plate anchored to a concrete body with tensile members extending through tensile conduits embedded in the concrete body;

FIG. 7 is a fragmentary sectional view of a cast concrete joint joining the base mat assembly and the ring beam with a shell element being welded to the ring beam;

FIG. 8A is a perspective view of a shell element;

FIG. 8B is a perspective view of a pair of shell elements which are welded together to form part of the outer shell with the shell elements also being welded to base plates of ring beam elements;

FIG. 8C shows a sectional view of a second embodiment of a shell made with two courses of lower and upper shell elements rather than using a single course as seen in the shell of FIG. 8B;

FIG. 8D shows a bottom view of the upper shell element which includes a base plate supporting two downwardly depending alignment pipes;

FIG. 8E shows a plan view of a top end cap plate of a lower shell element having two downwardly depending alignment sockets designed to receive the alignment pipes of the upper shell element;

FIG. 9A is a roof frame formed by radial and circumferentially extending wide flange beams;

FIG. 9B illustrates a typical welded connection between a tubular mounting beam of a roof assembly and a pair of roof elements;

FIG. 9C shows the roof assembly and attached seal being raised by air pressure within the outer shell so that the roof assembly can be welded to temporary gusset plates mounted to the top of the shell;

FIG. 9D is a fragmentary perspective view of the roof assembly and peripheral seal being raised to be temporarily welded to a gusset plate;

FIG. 9E is a fragmentary perspective view of a roof membrane of the roof assembly being temporarily welded to a gusset plate;

FIG. 9F is a fragmentary perspective view of the roof assembly permanently welded to the shell with the temporary gusset plates and seal having been removed and with a roof element attached to the roof assembly and shell;

FIG. 10A is a perspective view of a roof element frame used in casting a typical roof element; and

FIG. 10B is a perspective view of an exemplary roof element which is to be mounted to the roof assembly during construction of the roof.

DETAILED DESCRIPTION I. Overview

In a first embodiment, a full containment LNG tank 18 is shown in FIGS. 1 and 2. The LNG tank 18 comprises an outer secondary container, hereinafter “outer tank 20” surrounding an inner primary container, hereinafter “inner tank 22”. Inner tank 22 is intended to store LNG. Outer tank 20 is preferably a full containment tank in which both LNG and gas vapor are fully contained in the event of a leak from the inner tank 22.

In this particular embodiment, outer tank 20 is primarily made of precast reinforced concrete elements which reduce fire risk relative to using a primarily steel outer container or tank. The construction of outer tank 20 involves assembling precast reinforced concrete elements for a foundation 24, including a base mat assembly 100 and an outer ring beam 200, an outer wall or shell 300 and a roof 400. As an example of order of magnitude, outer tank 20 in this embodiment has a diameter of approximately 80 meters, an overall height (to the top of the dome of roof 400) of about 50 meters, and a height to the top of shell 300 of about 40 meters. Those skilled in the art of LNG facilities construction will appreciate that much larger or smaller tanks can be built and still use the design considerations described herein. However, tanks having diameters of at least 25 meters are particularly well suited to the construction methods described herein which utilize precast reinforced concrete elements. These elements ideally can be welded together to form structural welds thereby minimizing the number of cast in place concrete structural joints which have to be made to form an outer containment tank.

Ideally, inner tank 22 can be assembled at the same time as roof 400 is being built to save time and money in the overall construction of inner tank 22 and outer tank 20. Note in FIG. 1 roof elements 402 are being added ring by ring to a roof assembly 404 while inner tank 22 is being constructed. A construction access or shell opening 350 in shell 300 allows for materials to be carried into outer tank 20 so that inner tank 22 can be built as roof 400 is being constructed. After inner tank 22 is completed, special half height shell elements 302′ can be installed to close opening 350. An annular gap of approximately 1.5 meters exists between inner tank 20 and shell 300 to permit circumferential access to build inner tank 22 and to accommodate insulating materials (not shown) which are to be installed between outer and inner tanks 20 and 22.

Inner tank 22 can be built in any of a number of ways which may require welding processes to connect individual plates. For example, the plates can be joined using shielded metal arc welding (SMAW) and submerged arc welding (SAW) for 9% nickel tanks of the size described above, i.e. having a diameter greater than 25 meters. In this particular example, inner tank 22 can be friction stir welded (FSW) by plate at a time erection method. A course of plates at a time can be welded with the bottom (thickest) first and then subsequent (thinner) courses welded there above. Scaffolding (not shown) can be used as each course is added. Alternatively, inner tank 22 can be constructed course down either by supporting the tank from the roof to allow insertion of the next course or by supporting on jack stands to allow insertion of the next course. For the last two methods, the scaffolding need be erected only one time. By way of example and not limitation, another alternative method is to use coil material shaped to the curvature required for the tank. Coil tank building occurs from the top course down either using jacking or by roof supported. Expanded pearlite insulation is typically used in the annulus formed between inner tank 22 and outer tank 22. Although not shown, an under bottom insulating layer, which can be made of foam glass, is applied on the top of base mat assembly 100 with inner tank 22 prior to inner tank 22 being placed there on.

Foundation 24, i.e., base mat assembly 100 and ring beam 200, is designed to support inner tank 22 and outer shell 300. Outer shell 300 rests upon and transfers loads directly to outer ring beam 200 preferably through welded connections. A particularly significant load is the torsional load applied to ring beam 200 from outer shell 300. Base mat assembly 100 is secured to and transfers loads to outer ring beam 200 as well through a cast in place joint 270, which is best seen in FIG. 7. Depending on soil conditions, base mat assembly 100 and/or outer ring beam 200 can be supported upon compacted soil or else on piles, as will be described below. Roof 400 of tank 20 includes a roof assembly 404 comprising a skeletal steel frame 406 (FIG. 9A), a membrane 412 (FIG. 9B) of liner plates 410 and radially and circumferentially extending tubular mounting beams 414. Roof 400 is supported atop outer shell 300. Roof assembly 404 supports and is strengthened by a number of concentric rings or courses of precast reinforced concrete roof elements 402 and 402′. FIG. 2 shows a completed outer tank 22 with shell opening 350 being sealed and closed by special half shell elements 302′ which are permanently welded to the remainder of shell 300.

II. Base Mat Assembly 100

FIG. 2 shows base mat assembly 100 disposed within outer ring beam 200. A cast in place concrete connection 270 (not shown) is to be formed there between, as best illustrated in FIG. 7. In this particular exemplary embodiment, a large number of generally rectangular base mat elements 102 are connected together to form base mat assembly 100 having a contiguous top membrane 140 made from welded together steel liner plates 104. Membrane 140 ideally cooperates with similar membranes formed from liner plates on connection 270, outer ring beam 200, shell 300 and roof 400 to form an overall air tight vapor barrier within outer tank 20. As will be described later in greater detail, base mat assembly 100 and outer ring beam 200 preferably have discrete reinforced concrete elements that are clamped together by post-tensioned tensile members 150, 250, i.e., steel cables, and anchors 152, 252. Precast elements 102, 202 are then welded to together to form, respectively, base mat assembly 100 and outer ring beam 200.

A. Base Mat Elements 102

FIG. 3A shows a pair of juxtaposed base mat elements 102. FIG. 3B shows a generally rectangular liner plate 104 having a series of projections 106 and recesses 110. Liner plate 104 is preferably made of carbon steel although it can instead be made of other appropriate metals. The thickness is of liner plate 104 in this exemplary embodiment is about 10 mm. As an example, in this embodiment the size of liner plate 104 and base mat elements 102 are approximately 2 meters by 3 meters. A plurality of J-hooks 112 is welded to the lower surface of liner plate 104.

FIG. 3C depicts a base mat element 102 having a concrete body 114 cast about J-hooks 112 to anchor liner plate 104 to concrete body 144. Although not shown to simplify the drawing, formed in the concrete body 114 are two or more layers of reinforcing bars or welded wire fabric running in two generally perpendicular directions. For example, the reinforcing bars can be ASTM A 615 deformed carbon-steel bars for concrete reinforcement. Also formed within concrete body 114 is a plurality of laterally extending lower and upper tensile conduits 120, 122 and longitudinally extending lower and upper tensile conduits 124, 126 which extend in generally perpendicular directions to one another. These tensile conduits 120, 122, 124 and 126 can be HDPE (high density polyethylene) tubing or steel piping or other appropriate structural conduits.

Concrete ribs 130 and grooves 132 are also formed on the longitudinally and laterally extending edge surfaces of concrete body 114. Locating projections 106 and ribs 130 and receiving recesses 110 and grooves 132 allow a plurality of base mat elements 102 to be placed in interlocking juxtaposition as suggested in FIG. 2, FIG. 3A and FIG. 3D when base mat assembly 100 is being constructed. Note that concrete body 114 has downwardly and upwardly opening steps 134 and 136 so mating base mat elements 102 can interlock in the vertical direction as well as in the lateral and longitudinal directions.

The rows of tensile conduits are disposed below and above the central horizontal plane 144 of concrete body 114. These laterally and longitudinally extending tensile conduits are designed to receive tensile members 150 there through which allow multiple base mat elements 102 to be clamped together utilizing anchors 152 (FIG. 3D) upon the post-tensioning of the tensile members 150. Only some of the tensile members 150 are shown in FIG. 3D.

B. Construction of Base Mat Assembly 100

Base mat assembly 100 is constructed by juxtaposing base mat elements 102 in both first and second generally perpendicular directions as seen in FIG. 2. Locating projections 106 and concrete ribs 130 cooperate with receiving recesses 110 and grooves 132 on adjacent elements 102 to align base mat elements 102 in first and second generally perpendicular directions. Steps 134, 136 allow base mat elements 102 to interlock in the vertical direction as well.

Tensile members 150 are fed through tensile conduits 120, 122, 124 and 126 as base mat elements 102 are being juxtaposed to one another. After the mating surfaces on the base mat elements 102 are properly located relatively to one another, tensile members 150, i.e. cables, are post-tensioned and anchored by anchors 152 to clamp base mat elements 102 together. In this particular example, tensioning can be accomplished such as by using a Williams Strand Anchor System available from Williams Form Engineering of Belmont, Mich., USA. Those skilled in the art will appreciate that other tensioning systems can also be used to post-tension and anchor tensile members 150. After the tensioning is complete, anchors 152 are locked in place on tensile members 150 to maintain the tension in tensile members 150 with anchors 152 bearing upon base mat elements 102.

Liner plates 104 of each of base mat elements 102 are then welded together to form a part of a generally contiguous membrane 140 on the top of base mat assembly 100. As tensile conduits 120, 122, 124 and 126 are located above and below the central horizontal plane 144 of concrete body 114, the top and bottom surfaces of base mat elements 102 are held together and do not separate due to the post tensioning of tensile members 150 with anchors 152 clamping about base mat elements 102, preferably even under construction and operating loads applied to base mat assembly 100.

C. Mounting of Base Mat Elements on Piles

Base mat elements 102 can be assembled on a graded, level surface if the underlying surface or soil is sufficiently stiff. However, if the soil does not provide adequate support, base mat elements 102 can be mounted on piles 160. The upper portion of a typical pile 160 is seen in FIG. 4. Pile 160 includes a post 162 which is driven into the ground such as in soft soil 163. Four L-shaped brackets 164 can be bolted or welded to post 162 at an appropriate location with horizontally extending flanges 166 being at a predetermined height. Ideally, a laser positioning system is used to maintain all of the flanges 166 within a predetermined tolerance of height. Mounting base mat elements 102 on flanges 166 will then provide for easy welding of liner plates 104 together as the liner plates 104 should all be at about the same height. Reinforcing bars 170 extend upwardly from post 162 to facilitate later cast in place connections between piles 160 and base mat elements 102.

FIG. 5 depicts a couple of base mat elements 102 being mounted upon flanges 166 of a matrix of piles 160. Note in this case of utilizing piles 160 for support, base mat elements 102 are notched in their corners and along their long sides to mate with posts 162, as seen in FIG. 5. The corners and long sides of base mat elements 102 will be supported by six cooperating supporting flanges 166. Voids 174 are created by the intersection of the corners and along the sides of cooperating base mat elements 102. In the case where piles are used, base mat elements 102 can include reinforcing bar loops (not shown) which are embedded in concrete body 114 and extend laterally and longitudinally from concrete body 114. The loops will loop over vertically extending reinforcing bars 170. Concrete is cast in these voids 174 about the loops and reinforcing bars 170. Fitted liner plate 176 is anchored by J-hooks in the cast concrete so that liner plates 104 and 176 can be welded together to form generally continuous top membrane 140 on base mat assembly 100.

III. Outer Ring Beam 200

FIG. 2 shows a plan view of the foundation 24 including base mat assembly 100 and outer ring beam 200. The numbers of base mat and ring beam elements 102, 202 needed to construct these components will vary depending on the size of the desired outer tank 20 and the selected size of the various ring beam elements 202 and base mat elements 102. Various methods can be used to join base mat assembly 100 to outer ring beam 200. Preferably, ring beam elements 202 are clampingly held in a circumferential abutment by post-tensioning tensile members 250 and clamping anchors 252 about the ring beam elements 202. Then base plates 206 anchored within each of ring beam elements 202 are welded together to form an annular ring of such base plates 206. Similarly, inner liner plates 204 are also welded to one another to increase strength and stiffness and to form a membrane 244 of liner plates 204. Post-tensioned tensile members 250 and anchors 252 assist outer ring beam 200 in resisting bending, shear and torsional forces applied to ring beam 200 by outer shell 300 and by base mat assembly 100 to maintain ring beam elements 202 in the proper abutting relationship with respect to one another.

A. Ring Beam Element 202

FIGS. 6A-C illustrate an exemplary embodiment of a ring beam element 202. In plan view, FIG. 6A shows a pair of ring beam elements 202 interlocking with one another when placed in side by side position. FIG. 6B shows an inner liner plate 204 secured to a thick base plate 206. Liner plate 204 and base plate 206 are preferably made from carbon steel. Liner plate 204 is approximately 10 mm thick and base plate 206 is about 50 mm thick in this example. J-hooks 212 are welded to the bottom sides of liner plate 204 and base plate 206. Projection 214 and recess 216 are formed on the edges of liner plate 204. FIG. 6C illustrates that ring beam element 202 includes a concrete body 220 in which liner plate 204, base plate 206 and J-hooks 212 are embedded. An outer radial surface 222 and an inner radial surface 224 define what will be part of the radially outer and inner surfaces of ring beam 200. A plurality of locating concrete ribs 226 and receiving recesses 230 allow juxtaposed ring beam elements 202 to be cooperatively interlocked with one another, as seen in FIG. 2 and FIG. 6A, to form outer ring beam 200.

Also formed within concrete body 220 are circumferentially extending lower and upper tensile conduits 232, 234. In this particular embodiment, there are four such tensile conduits 232, 234. Two of these tensile conduits 232 are generally arranged below and two conduits 234 are arranged above the horizontal center plane 236 of ring beam element 202. Also, two of the tension conduits 232, 234 are arranged beneath base plate 206 and two are located radially inwardly beneath liner plate 204 closer to the inner radial surface 224. These cooperating locations of tensile conduits 232, 234 allow tensioning members 250, i.e., cables, to pass through each of ring beam elements 202 and assist in counter balancing the bending, shearing and torsional loads applied to base plate 206 by outer shell 300. Also, tensile members 250 and anchors 252 cooperate to clamp about ring beam elements 202 to prevent the abutting ring beam elements 202 from displacing with respect to one another. In this particular example, high density polyethylene (HDPE) tubing is used to form the tensile conduits 232, 234 in concrete body 220. Of course, the tensile conduits could be made of other suitable materials such as steel or other structurally strong materials. Downwardly and upwardly opening steps 238 and 240 allow elements 202 to be vertically interlocking as well. Also, located within concrete body 220 is a plurality of reinforcing bars (not shown) which are conventional for adding tensile strength to cast concrete bodies. Extending radially inwardly are reinforcing bars or J-hooks 242 which are later to be included in cast concrete connection 270 which connects base mat assembly 100 with ring beam 200.

FIG. 6C illustrates a ring beam element 202 having four circumferentially extending tensile conduits 232 and 234 extending there through. This embodiment may be appropriate for cases where outer ring beam 200 is supported by stiff underlying soil or surface. In the event that a softer underlying soil is available at a site, it may be preferably to include an additional pair of tensile conduits located radially outboard of the locations where the tensile conduits 232, 234 are positioned in FIG. 6C. This allows additional clamping force to be applied across the ring beam elements 202. FIG. 7 shows a ring beam element 202 with three pairs of vertical spaced apart tensile conduits 232 and 234. Also, as discussed above with respect to base mat elements 102, ring beam elements 202 can also be mounted on leveled flanges of piles to insure proper support and vertical alignment between ring beam elements 202 which are to have liner plates 204 and base plates 206 welded together.

B. Construction of Outer Ring Beam 200

Ring beam elements 202 are juxtaposed with respect to one another with locating concrete ribs 226 of one ring beam element being held within locating recess 230 of the adjacent ring beam element. Similarly, cooperating steps 238 and 240 assist in vertical alignment between ring beam elements 202. Tensile members 250 are placed through tensile conduits 232 and 234 as the ring beam elements are being positioned adjacent one another. As seen in FIGS. 1A and 1B, at four locations along the circumference of outer ring beam 200, there are pairs of special abutting ring beam elements 202′ through which the ends of tensile members 250 extend. Tensile members 250 are tensioned utilizing tensioning devices 260 and anchors 252 are anchored about ring beam elements 202. In this particular example, tensioning can be accomplished such as by using the aforementioned Williams Strand Anchor System. Those skilled in the art will appreciate that other tensioning systems can also be used to post-tension and anchor tensile members 250.

After the individual ring beam elements 202 are aligned and clamped together using tensile members 250 and anchors 252, base plates 206 are welded together along their radially extending abutting edges to form an annular ring which strengthens outer ring beam 200. Finally, liner plates 204 on ring beam elements 202 are also welded together along their radially extending edges to form a continuous membrane 244 on the radial inner side of outer ring beam 200, as best seen in FIG. 8B.

C. Connection between Base Mat Assembly 100 and Ring beam 200

FIG. 7 illustrates an example of how a base mat assembly 100 can be connected to the outer ring beam 200. Concrete is cast in place about reinforcing bars 142, 242 extending outwardly from adjacent base mat elements 102 and radially inwardly from ring beam elements 202 to form a cast in place joint 270 between the base mat assembly 100 and outer ring beam 200. T-7299 Liner plates 272 are cut to the necessary size and shape to mate between liner plates 104 of base mat assembly 100 and inner liner plates 204 of outer ring beam 100. J-hooks 274 are welded to the bottom side of liner plates 272. Concrete is cast in place between base mat assembly 100 and base ring 200 to create a concrete body 276 anchoring liner plates 272 and J-hooks 274, 142 and 242 in the cast concrete to form joint 270. Although not shown, appropriate reinforcing bars will also be positioned in the space to be occupied by joint 270 prior to the concrete being cast to enhance tensile strength of joint 270.

IV. Outer Shell 300

FIG. 8A shows a perspective view of a typical shell element 302. FIG. 8B depicts a pair of shell elements 302 which are welded together and form a part of outer tank wall or shell 300. To reduce construction time for outer tank 20, shell 300 is built using precast reinforced concrete shell elements 302 that are joined by welding shell elements 302 together to form a generally annular, tapered thickness annular shell 300. Shell elements 302 are thicker at their bottoms to handle greater loads applied there to near ring beam 100 as compared to the loads imposed by roof 400. Preferably, there is minimal or no need to cast in place any significant structural connections to join shell elements 302 together or to ring beam 200 or to roof 400. Shell elements 302 will be precast to the number, widths and heights required for the size of outer tank 22. Typically, the inner diameter of outer tank 20 will be about 3 meters larger in diameter than inner tank 22 to allow for construction of inner tank 22 and to provide for insulation in the annulus between the tank shells. Precasting of shell elements 302 can be started as soon as the tank diameter and height are known.

A. Shell Element 302

Each of shell elements 302 includes a pair of tapered carbon steel side plates 304 and a carbon steel liner plate 306 forming a generally U-shaped steel cross-section. In this example, the liner plate 306 is about 10 mm thick and side plates 304 are about 25 mm thick. The width of side plates 304 is about 800 mm at their bottom and 400 mm at the top providing the tapered shape to shell element 302. A base plate 312 connects side plates 304 and liner plate 306 at the bottom of shell element 302. Numerous reinforcing bars 310 are welded to and extend from side plate 304 to the opposing side plate 304 and from base plate 312 to top end cap 314. Base plate 312 is about 50 mm in thickness in this example. A steel end cap plate 314, also about 50 mm thick, is welded to side plates 304 and steel liner plate 306 at the top of shell element 302. Concrete is cast in place in the U-shaped volume defined by side plates 304 and steel liner plate 306 and about reinforcing bars 310 and J-hooks 316 to form reinforced concrete body 318. Liner plate 306 acts as a vapor/gas barrier

B. Constructing Shell 300

Shell 300 is constructed by arranging shell elements 302 vertically upon outer ring beam 200. Initially, base plates 312 are arranged radially slightly outside of their final position for welding with shell elements 312 being slightly radially spaced apart. Shell elements 302 are then moved radially inwardly until all base plates 312 and side plates 304 are brought into abutment at the proper radial position atop of base plates 206 of ring beam 202. Erection gear will be used to finally align and pull the side plates 304 closely together to begin welding.

As seen in FIG. 8B and FIG. 1A, vertically extending weld joints 320 are made on the inside and outside of abutting side plates 304. Weld joints 320 ideally are designed to be strong enough to carry the loads on shell 300 without any or little other significant reinforcement, in particular, the need for post tensioning or utilizing any type of significant cast in place joints. End cap plates 314 are welded to one another forming weld joints 322 and creating a continuous upper ring on shell 300. As best seen in FIG. 7, base plates 312 of shell 300 and base plates 206 of ring beam 200 are welded together along the inner and outer radial surfaces on base plate 312 to form base plate welds 324. Shell elements 302 are splice welded to accommodate load transfer even under earthquake or missile impact. If additional strength is needed, shell elements 302 may be designed to be mechanically interlocking with one another as well as being joined by welds such as weld 320. Ideally, the welded connections between shell elements 302 will be comparable in strength to conventional cast in place connections formed between shells.

All of shell elements 302 are permanently welded together to form shell 300, with the exception of 2-4 special half shell elements 302′. Half shell elements 302′ are similar to shell elements 302 except they are only about half the height of regular shell elements 302. These special shell elements 302′ will be temporarily sealed with shell opening 350 to accommodate air raising of roof assembly 404, as suggested in FIG. 9C. These special half shell elements 302′ are removed after roof assembly 404 is attached to shell 300 to provide access so that inner tank 22 can be concurrently erected as reinforcing roof elements 402 and 402′ are welded to roof assembly 404. When inner tank 22 is completed, along with insulation added in the annulus formed between inner tank 22 and outer tank 20, the special half shell elements 302′ are permanently welded to the remainder of shell 300 to seal shell opening 350, as shown in FIG. 1B. Although not shown in FIGS. 1A and 1B, manhole access openings will be provided in roof 400 to provide access to perform welds on the interior of outer tank 20. Also, the manhole access openings allow future maintenance to be performed within outer tank 20.

As an alternative to permanently closing construction opening 350, liner plates can be welded to close shell opening 350. Then reinforcing bars and steel rib stiffeners can be placed in a form and concrete can be cast in place to form a cast in place shell element similar to that used in the design of the precast shell elements 302. Shell 300 should have great strength when completed due to the welded connections 320 on the inner and outer edges of side plates 304 which cooperate to form a large number of radially extending stiffeners arranged around the circumferential periphery of outer tank 20.

C. Alternate Shell Design

FIGS. 8C, 8D and 8E show an alternative shell design. Rather than using a single course of precast reinforced concrete shell elements 302 to form shell 300, multiple courses of similar upper and lower shell elements 302 a and 302 b are utilized. Those skilled in the art of LNG tank construction will appreciate that any number of courses of shell elements could be used to form a shell. However, shorter shell elements are easier to transport and move but require additional welds as compared to using a single course of shells to construct a shell.

Lower shell element 302 a includes a concrete body 316a to which a pair of circumferentially spaced apart side plates 304 a, liner plate 320 a, upper end cap plate 314 a and lower base plate 312 a are anchored or embedded (anchors and reinforcement bars not shown). Base plate 312 a is welded by welds 324 a to base plate 206 of ring beam 200. Upper end cap plate 314 a has two downwardly depending sockets 362 formed therein.

Upper shell element 302 b includes a concrete body 316 b to which a pair of circumferentially spaced apart side plates 304 b, liner plate 320 b, upper end cap plate 314 b and lower base plate 312 b are anchored (anchors and reinforcement bars not shown). Two downwardly depending and circumferentially spaced apart alignment pipes 360 are welded to lower base plate 312 b. As seen in FIG. 8C, alignment pipes 360 can be used to align and guide an upper shell element 302 b as it is positioned upon lower shell element 302 a. Also, alignment pipes 360 held within alignment sockets 362 provide mechanical interlocking between shell elements 302 a and 302 b. Base plate 312 b is welded by horizontal welds 364 to end cap plate 314 a of lower shell element 302 a along their inner and outer radial edges. A horizontal extending cutout 364 on the outer radial edges of shell elements 302 a and 302 b permits the outer radial weld 364 to be easily made. After welding, cutout 364 is filled with grout for fire protection.

V. Roof 400

Referring in general now to FIGS. 9A-F, roof 400 has a roof assembly 404 which includes a skeletal roof frame 406 (FIG. 9A), liner plates 410 covering roof frame 406 (FIG. 9B and 9C) to form a generally air tight membrane 412 and tubular mounting beams 414 welded atop membrane 412. In this exemplary embodiment, after a temporary skirt or seal is attached to its circumferential periphery, roof assembly 404 is air lifted to the top of shell 300 from upon base mat assembly 100 by pressurizing the inside of shell 300, as suggested in FIG. 9C. Roof membrane 412 of roof assembly 404 is welded to circumferentially spaced apart steel gusset plates 450 (FIGS. 9D and 9F) that are temporarily attached atop end cap plate 314. This prevents the roof seal from blowing out and allows weld joints to be made between the roof membrane 412 and gusset plates 450. Air pressure is maintained while welds are made between end cap plate 314 and roof membrane 412.

Once enough weld joints have been formed to safely support roof 400, the pressurization within shell 300 is removed. Personnel can then access the inside of the outer tank 20 to construct inner tank 22. Additional welding is done to complete the circumferential weld joint between the roof membrane 412 and shell membrane 340. Special extra strength roof elements 402′ are attached to shell 300 and roof frame 404 forming the radial outermost course of roof elements 402. Then typical roof elements 402 are attached to roof frame 404 with the radial outermost course of roof elements 402 being attached first. Then the successive next radial outermost course of roof elements is added until all the rings of roof elements 402 are in place forming roof 400.

A. Roof Assembly 404

Roof assembly 404 is built on the ground in this particular preferred exemplary embodiment. Roof frame 406 comprises a plurality of radially and circumferentially extending wide flange beams 420, as best seen in FIG. 9A. In this particular exemplary embodiment, eight concentric rings of openings 422 a-h are formed between the wide flange beams 420. Wide flange beams 420 used in the radial direction are W18×76 in the center and gradually increase in size to W18×143 near the outer radial ends. In the circumferential direction each row of wide flange beams 412 has the same section size such as W18×40, W18×50, W18×76 and W18×97 with the larger wide flange beams being used nearer the outer radial edge. Of course, other sizes of wide flange beams can be used, particularly if outer tank 20 is to be of a smaller or larger size than suggested in the present example.

FIG. 9B shows a typical joint construction for roof assembly 404. Liner plates 410 are appropriately sized and are welded to the top flange 424 of wide flange beams 420 to cover openings 422 and thus provide a generally air tight roof membrane 416. A tubular mounting beam 414 is shown which is welded atop of adjacent liner plates 410. Tubular mounting beams 414 generally run in the same direction as the web of the underlying wide flange beams 420, i.e., radially and circumferentially. As can be seen in FIG. 9B, roof elements 402 are designed to mount and be welded to tubular mounting beams 414, as will be described in greater detail below.

B. Raising and Mounting Roof Assembly 404 to Shell 300

FIG. 9C shows a roof assembly 404 being raised by air pressure along an outer tank wall or shell element 302. The three dots shown adjacent shell element 302 indicates that shell 300 is comprised of a large number of such shell elements 302, which are not shown. A temporary skirt or seal 424 is attached around the circumferential periphery of roof assembly 404. FIG. 9D shows a fragmentary sectional view of roof assembly 404, seal 424, and end cap plate 314 and liner plate 306. Air pressure is applied to seal 424 from within shell 300. Seal 424 bears against liner plate 306 of shell 300 to minimize the loss of air there between. A temporary gusset plate 450, one of 240 such gusset plates 450 located around the periphery of the ring of end cap plates 314 in this example, is temporarily welded to end cap plate 314 and extends radially inwardly.

Referring now to FIG. 9E, when roof assembly 404 is raised to the appropriate height, welds 452 are made between liner plates 410 and gusset plates 450. An outer portion of a circumferential weld 454 is then made between end cap plate 314 and roof liner plates 410 to form an air tight seal there between joining shell membrane 340 and roof membrane 412 of roof assembly 404. Ideally, weld 454 is a full fusion, full penetration weld requiring welding from both outside and inside outer tank 22. When sufficient welds have been made to safely secure roof assembly 404 to shell 300, air pressure can be removed from shell 300. Then, seal 424 and gusset plate 450 are removed.

Half shell elements 302′ are removed to provide access from within shell 300. Referring now to FIG. 9F, the inner portion of circumferential weld 454 is then completed between end cap plate 314 and liner plates 410 to form an air tight seal there between joining shell membrane 340 and roof membrane 412. A series of circumferentially spaced apart permanent flange extension plates 462 are welded between liner plate 306 and beneath the lower flanges of wide flange beams 420. Also, web extension plates 464 are welded on to the web of wide flange beam 420 and to liner plate 306. On the outside of roof assembly 404, extension tubular mounting beams 414 a and 414 b are, respectively, welded atop liner plates 410 and end cap plate 314 extending radially from mounting beam 414. With tubular extensions 414 a and 414 b in place, the outermost row of roof elements 402′ can then be welded to shell 300 and to roof assembly 404, as will be described in greater detail below.

C. Roof Elements 402 and 402′

Roof elements 402 and 402′ are mounted to roof assembly 404 after roof assembly 404 has been affixed by weldments to shell 300. FIG. 10A shows a typical roof element frame 428 which includes four L-shaped brackets 432 welded together to form a generally open rectangular frame. Depending inwardly from brackets 432 are a plurality of J-hooks 434. Reinforcing bars 436 extend laterally and longitudinally between opposing brackets 432.

Roof element frame 428 is used to construct precast reinforced concrete roof elements 402, as seen in FIG. 10B. Roof element 402 includes a cast concrete body 440 which surrounds J-hooks 434 and reinforcing bars 436 and supports roof element frame 428. The L-shaped brackets 432 are located on all four sides of roof element 402 to provide attachment to tubular mounting beams 414 of roof assembly 404 in the case of typical roof elements 402. The size of each roof element 402 is generally between about 2 meters to about 3 meters in width and between about 5 meters to about 6 meters in length in this exemplary embodiment. Of course, other size roof elements could be selected and will depend on the size of outer tank 22.

For the outermost concentric course of roof elements 402′ which secure to both roof assembly 404 and to shell 300, the design is slightly different from that of roof elements 402 disposed on the inner radial concentric courses. As shown in FIG. 9F, concrete body 440′ in this example tapers from a thinner 150 mm on its inner radial end to a maximum thickness of 300 mm adjacent its radial outmost end. The typical concrete body 440 of a typical roof element 402 has a constant thickness of about 150 mm. Second, roof element 402′ includes L-shaped bracket 432 on only three edges with the radially outermost fourth side instead having an overhang 470 sized and shaped to fit over end cap plate 314. Provided in concrete body 440′ are four access openings 472 (two seen in FIG. 9F) which provide access to four studs 474 which are to be stud welded to end cap plate 314.

D. Mounting of Roof Elements 402 to Roof Assembly 404

The outer concentric ring or course of roof elements 402′ are located adjacent shell 300 and this outer ring of roof elements 402′ is first welded to roof assembly 404 and to shell 300. This outer ring of roof elements 402′ adds significant strength to roof 400. Subsequently, second through eighth courses of roof elements 402 are sequentially welded to roof assembly 404 starting from the radially outermost course and then each concentric course of roof elements is added until all rows of roof elements 402 are in place to form a complete roof 400 such as seen in FIG. 1B. FIG. 1A shows two courses of roof elements 402′, 402 mounted to roof assembly 404.

L-shaped brackets 432 of roof elements 402 are mounted atop and are welded to tubular mounting beams 414 creating welds such as the weld joint 436 seen in FIG. 9B. In the case of special roof elements 402′, roof elements 402′ are welded to tubular mounting beams 414, 414 a and 414 b. Studs 474 are placed in access openings 472 and are stud welded to end cap plate 314. The thickness of tubular mounting beams 414 is less than the thickness of concrete bodies 430, as seen in FIG. 9B. Any voids formed between any of concrete bodies 440, 440′ of the roof elements 402, 402′ are grouted in place to form concrete joint 442 to provide fire protection to roof 442. An example of a concrete joint 442 is shown in FIG. 9B.

While in the foregoing specification this invention has been described in relation to certain preferred embodiments thereof, and many details have been set forth for purpose of illustration, it will be apparent to those skilled in the art that the invention is susceptible to alteration and that certain other details described herein can vary considerably without departing from the basic principles of the invention. For example, any one or more of base mat assembly 100, ring beam 200, outer shell 300 or roof 400 could be constructed using conventional cast in place construction techniques while the other components are constructed using the precast elements as described herein. As another example of an alternative embodiment, the roof assembly could be directly built atop shell 300 such as by using a crane. In this instance, roof assembly 404 would not have to be airlifted and attached atop shell 300. However, ideally even in this embodiment, the shell elements 402 could be welded to the roof assembly concurrently with the construction of the inner tank 22 to save construction time on building outer tank 20 and inner tank 22.

While beam ring 200 described in the above particular exemplary embodiment was used in conjunction with a full containment LNG tank, a beam ring could certainly be used in other applications. For example, the ring beam could be used in cases where neither a liner plate or thick structural base plate is necessary. In this case, the ring beam need only comprise ring beam elements having precast reinforced concrete bodies with embedded tensile conduits which receive tensile members there through so that the tensile members may be tensioned and anchored by anchors to form the beam ring. Although not required, preferably the ring beam elements would be interlocking with one another. 

1. A method of constructing a base mat assembly for supporting a primary container storing liquefied natural gas tank, the method comprising: providing a plurality of precast base mat elements, at least some of the base mat elements having: a concrete body having first and second tensile conduits extending there through and a top membrane plate anchored in the concrete body; juxtaposing a plurality of the base mat elements; inserting tensile members through the tensile conduit members of the base mat elements and tensioning and anchoring the tensile members about the base mat elements to clamp the base mat elements together; and welding the membrane plates of the base mats together to form a generally horizontally extending base mat assembly with a generally contiguous membrane.
 2. The method of claim 1 wherein: the first and second tensile conduits in a concrete body extend in generally perpendicular first and second directions to one another; and the tensile members clamp the base mat elements in generally perpendicular directions.
 3. The method of claim 2 wherein: the first and second conduits include at least two conduits in each of the first and second directions, the at least two conduits being located above and below the horizontal center plane of the concrete body.
 4. The method of claim 1 further comprising: interlocking the concrete bodies of the at least two base mat elements to one another prior base mat elements being clamped together by the tensile member and anchors, the concrete bodies having interlocking surfaces.
 5. The method of claim 4 further comprising: interlocking the concrete bodies in a horizontal direction.
 6. The method of claim 4 further comprising: interlocking the concrete bodies in the vertical direction.
 7. The method of claim 4 further comprising: interlocking the concrete bodies in a horizontal direction and in the vertical direction.
 8. The method of claim 1 further comprising: mounting the base mat elements upon piles to provide enhanced stiffness to the base mat assembly.
 9. A base mat assembly for supporting a primary container storing liquefied natural gas, the base mat assembly comprising: a plurality of precast reinforced concrete base mat elements, at least two of the base mat elements including a concrete body having first and second tensile conduits extending there through in first and second generally perpendicular directions and a liner plate anchored in the concrete body; and a plurality of post-tensioned tensile members extending through the first and second conduits and a plurality of anchors which are anchored on the tensile members and clamp the base mat elements together in first and second generally perpendicular directions; wherein the liner plates are welded together to form a generally horizontally extending base mat assembly with a generally contiguous membrane on top the concrete bodies.
 10. The base mat assembly of claim 9 wherein: the concrete bodies of the base mat elements have interlocking surfaces; and the concrete bodies are interlocked with one another utilizing the interlocking surfaces.
 11. The base mat assembly of claim 9 wherein: the base mat elements interlock in first and second horizontal directions.
 12. The base mat assembly of claim 11 wherein: the base mat elements interlock in a vertical direction.
 13. The base mat assembly of claim 11 wherein: at least one of the first and second tensile conduits extends horizontally above and at least one extends horizontally below the horizontal center plane of the concrete bodies of the base mat elements.
 14. The base mat assembly of 13 further comprising: a plurality of piles driven into the earth which support the base mat elements and provide additional stiffness to the membrane formed by the liner plates.
 15. A base mat element for constructing a generally horizontally extending base mat assembly, the base mat element comprising: a concrete body having first and second conduits extending there through in first and second generally perpendicular directions and a generally planar and horizontally extending liner plate anchored in the concrete body; wherein a plurality of the base mat elements may be juxtaposed one another with tensile members extending through the first and second tensile conduits with anchors anchoring about the tensile members to clamp the base mat elements together with a generally planar membrane surface being formed by the clamped together base mat elements.
 16. The base mat element of claim 15 wherein: the concrete body has interlocking surfaces formed thereon such that the interlocking surfaces of a plurality of such base mat elements can interlock with one another when the base mat elements are juxtaposed one another.
 17. The base mat element of claim 15 wherein: at least one of the first and second tensile conduits is located above and one below the center horizontal plane of the concrete body so that when tensile members are passed through and anchored about the base mat elements, the clamped base mat elements resist displacing vertically relative to one another.
 18. The base mat element of claim 15 wherein: the base mat element has interlocking surfaces which allow a plurality of the base mat elements to be juxtaposed one another and interlocked.
 19. The base mat element of claim 18 wherein: the interlocking surfaces allow a plurality of the base mat assemblies to be interlocked in first and second horizontal directions and in a vertical direction. 